Some 5,000 people, including Liberal Democratic Party leader Vladimir Zhirinovskii and Chechen President Ramzan Kadyrov, gathered in the cathedral to celebrate the start of the holiday.
Russian President Vladimir Putin attended a Christmas service in the Siberian city of Yakutsk, where he was on a working visit yesterday.
Other Orthodox Christians, Armenians, and Eastern Rite followers are marking Christmas around the world.
Christmas is 7 January for Christians in the Holy Land, Russia, and other churches that use the Julian calendar.
The Gregorian calendar is used by Catholics and Protestants and commonly in secular life.
RFE/RL's World: 2005 In Pictures
A slideshow of images related to the top news stories of 2005 from throughout RFE/RL's broadcast region with links to RFE/RL's reporting.